Each image memory or ``channel'' has independent scroll and zoom capabilities as well as an intensity transformation table which can be used like a colour look--up table and can also be used to change the output values that are fed to the look--up tables in the video output controller. This allows fast displays of log or histogram equalized images without having to reload the entire image.
One image channel is designated as the graphics (or overlay) channel. This also has its own zoom and scroll capabilities. In addition, the colour of the overlay can be selected. In MIDAS the last available channel is always used as the graphics channel. Also an alphanumeric memory is associated with the image display station.
The video output controller (VOC) selects which image memory is to be displayed on which colour channel as well as performing the task of overlaying the graphics plane. It also takes care of integrating the cursor and alphanumeric data into the video output. Finally, the VOC supports Split Screen mode where parts of 2 or 4 image channels are displayed together on the screen.